The present invention relates to a signal transmission circuit which is well suited for application to audio equipment, and the like.
Variable impedance circuits, such as signal transmission circuits in which the impedance of a signal transmission path is varied by a current control, have been known from U.S. Pat. Nos. 3,761,741 and 4,220,875. Prior to the present invention, we have studied the known variable impedance circuits. As a result of these studies, it has been found that the current consumption in these circuits increases sharply when the impedance is varied to a small value. That is, we have found that the current consumption increases in inverse proportion to the impedance.
On the other hand, a switched capacitor filter, which is a sampled value filter, has received quick acceptance for use as a monolithic filter in recent years, as reported in Proceedings of IEEE 67. 10. page 1403, October, 1979, "Application of CCD and Switched Capacitor Filter Technology" by Hewes et al. In a switched capacitor filter, an impedance is equivalently formed by the capacitance of a capacitor and the frequency of a clock signal applied to switching means connected to the capacitor.